These studies examine the postnatal development of neurons in trigeminal pain pathways in the dorsal horn of the medulla (trigeminal nucleus caudalis) with Golgi and electron microscopic techniques. They compare in newborn kittens the maturity of the two major interneurons in the substantia gelatinosa (the stalked cell and the islet cell) with their adult counterparts. They follow the development sequences in the formation of the dendritic and axonal arbors of both stalked cells and the islet cells and examine how these interneurons eliminate excess dendrites as well as how they provide additional plasma membrane for their elongating dendrites and axons. These studies also determine which components of the synaptic circuitry in trigeminal pain pathways are present at birth and which develop postnatally.